Huettel, Scott A., Allen W. Song, and Gregory McCarthy. Functional Magnetic Resonance Imaging. Sinauer Associates, 2009.
[Chapters 1 to 5 describe the general physical principles of MRI in an accessible way to non-physicists, while chapters 6 and 7 explain the physiological and physical basis of the BOLD contrast mechanism comprehensively.]

MR physics

Haacke, E. Mark, Robert W. Brown, Michael R. Thompson, Ramesh Venkatesan, and Norman Cheng. Magnetic Resonance Imaging: Physical Principles and Sequence Design. Wiley John + Sons, 2013.
[The reference book on MR physics, also known as the ‘green Bible’, as it used to be green in the most recent 1999 edition. Parts of it need a lot of physical intuition, and it does not focus particularly on fMRI. However, it is well-written and also covers the mathematical prerequisites needed to understand MRI.]

Bernstein, Matt A., Kevin F. King, and Xiaohong Joe Zhou. Handbook of MRI Pulse Sequences. Elsevier, 2004.
[With mathematical rigour, fundamental concepts of image generation and reconstruction, including artefacts, are explained in this book. Very clearly written, but the main audience are researchers interested in writing pulse sequences for the MR scanner themselves.]